The effects of solar radiation on concentrations and microbial utilization of various carbon and nitrogen compounds were studied in July in a thermally stratified lake in southern Sweden. Exposure of bacteria‐free water to natural sunlight in the surface of the lake for 7 h around noon led to higher concentrations of inorganic carbon (39–80%), amino acids (0–23%) and carbohydrates (0–15%), while lower concentrations of monosaccharides (0–38%), nitrate (0–23%) and urea (0–27%) were measured. Ammonium was unchanged. Lake bacteria were inoculated into the irradiated water and into water that had not been exposed to solar radiation (dark controls). The bacterial production was 35 to 80% higher during exponential growth (20 h after inoculation) in the irradiated samples than in the controls. The bacterial utilization of specific carbon and nitrogen compounds in the irradiated samples differed from that in the controls, but the changes in the epilimnion and the hypolimnion varied. Dominant nutrients to the bacteria were carbohydrates, amino acids, glucose and ammonium. In the controls a release of combined amino acids (epilimnion) or carbohydrates (hypolimnion) occurred. An apparent non‐biological removal of urea in the irradiated hypolimnion samples was found, since the microbial urea degradation was only 1% of the reduction in concentration. Our results suggest that biogeochemical cycling in natural waters is influenced by sunlight, due to changes of microbially available components that were not reported previously, including amino acids, carbohydrates, nitrate and urea.
Bacterial abundance and production in a vertical profile in Lake Kariba (17 °S), Zimbabwe, were affected by solar irradiance. At the surface, 1 .87 x 109 bacteria 1 -1 were found and abundance peaked at 10 m (2 .5 x 109 bacteria 1 -1 ), then decreasing with depth . Bacterial production at the surface (0 .145 sg C 1 -1 h -1 ) was nearly four times less than production at 10 m although bacterial numbers were only 26% less . Thus, bacterial production per cell was lower at the surface than deeper down, suggesting that bacterial production is inhibited at the surface .Bacterial production in GF/F filtered lake water in Whirl Pack TM bags showed an exponential decrease down to 3 m depth . The inhibition was well in accordance with light extinction in the UV region . Phosphatase activity was low in light exposed bags compared to dark, indicating photolysis of extracellular enzymes, or phototransformation of recalcitrant DOM, which substitutes enzyme activity. Hypolimnetic enzyme activity was less affected by solar light than epilimnetic .
Summary 1. The effects of solar radiation on bacterial and fungal growth on aquatic macrophyte detritus were studied in a microcosm experiment. Senescent leaves of Phragmites australis were incubated for 63 days in shallow water in the shade under photosynthetically active radiation (PAR) together with ultraviolet radiation, or under filters removing either ultraviolet B (UVB) or both UVB and ultraviolet A (UVA). 2. Bacterial abundance and bacterial 3H‐leucine incorporation in the water were measured, together with α‐ and β‐D‐glucosidase activity. In addition, bacterial abundance and fungal biomass associated with the litter were measured. 3. The results indicate that both PAR and UVA affect the micro‐organisms involved in the decomposition of leaf litter. The α/β‐D‐glucosidase activity ratio was less than one in irradiated and more than one in shaded microcosms, suggesting a change in the substrate dissolved organic matter composition towards more β‐ than α‐glycosidic linkages as a result of solar radiation. 4. Microcosms receiving UVB displayed a significantly higher β‐D‐glucosidase activity than shaded microcosms, and those exposed to PAR or PAR + UVA, demonstrating the potential importance of UVB radiation. 5. The free‐living bacteria tended to be dominated by filamentous forms in microcosms subject to solar radiation, especially PAR, and attached microbial communities showed a greater tendency to be dominated by bacteria in irradiated microcosms than in shaded microcosms.
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